High-temperature devices that can convert chemical energy of a fuel such as hydrogen directly into electrical energy at high efficiency and low or no air pollution are of great commercial interest. Such devices include high-temperature electrochemical devices, solid oxide fuel cells (SOFC), and other similar structures such as interconnects. Electrochemical devices having multiple components, such as, for example, solid oxide fuel cell (SOFC) stacks, syngas membrane reactors, oxygen generators and the like require seals to separate the various gaseous components [e.g., H2 (fuels) and O2 (oxidants)] and to prevent the streams from mixing with each other. Mixing of the gas streams has a variety of negative consequences, depending upon the type of device and the composition of the gaseous streams. One major problem resulting from mixing of such gases is the possibility of thermal combustion of the gases and the resulting failure of the device. Thus, to ensure high efficiency and to maintain the stack structural integrity, seals are needed to separate the various gaseous components (fuels and oxidants). Such seals must be non-conducting, have chemical, mechanical, and/or thermal compatibility with other structural components of the devices. The seals must also exhibit very low operational leak rates in severe (oxidizing, reducing, and humid) environments as well as long-term thermal cycling stability at elevated temperatures.
Continued thermal cycling at high operating temperatures up to about 850° C. results in increased leak rates in mica-based compressive seals, a consequence of damage resulting from fragmentation, cleavage, micro-stress fractures, and similar processes to the microstructure of the mica substrate matrix. Such damage introduces leak paths or void spaces (interstices) that are continuous in three dimensions. High leak rates and thermal cycling instability of current seals under routine high temperature operation represent two of the most challenging hurdles remaining for significant advancement to be made in the long-term success of high-temperature devices, including electrochemical devices such as SOFCs, and/or toward developing and/or deploying long-term viable components in other similar high temperature devices.
Accordingly, there remains a need to provide advanced seals and sealing structures having low effective leak rates and superior thermal cycling stability such that at the operating temperatures of these high temperature devices (up to about 850° C.), gaseous leaks (e.g., H2 gas into the air stream or vice versa) do not cause undesirable local heating leading to structural or functional failure of the device.